Human immunodeficiency virus (HIV) penetrates the brain early in the course of disease, and frequently causes dementia and other neurologic manifestations. The mechanism(s) underlying the neuropathologic effects of HIV infection are poorly understood. Prominent infection has been documented in microglia and macrophages, and less widespread infection of astrocytes, endothelial cell lines and even neurons has also been noted. Pathogenesis is likely to involve both direct cytopathic effects on target cells and indirect pathways driven by secreted cellular factors that alter neuronal viability. Elucidation of the mechanisms of infection for each cell type is a key step toward uncovering these pathogenic pathways as a foundation for informing new therapies to retard neurologic manifestations. The principal determinant of cellular tropism is the variable envelope glycoprotein gp120, which typically engages CD4 and particular chemokine receptors (coreceptors) expressed on cell surfaces. Relatively little is known about the distinct infection properties of viruses that infect the brain or about the specific routes of cellular infection by these viruses. A working hypothesis of neuropathogenesis is that gp120 determinants of specific strains of HIV promote entry into the CNS and infection of select grain cells through the use of CD4 in conjunction with a restricted subset of coreceptors. We have recently found that functional viral receptor complexes can also be formed using components expressed on separate cell surfaces, and this type of trans-receptor pathway may permit infection of CD4-negative brain cells types such as astrocytes that are vital to maintaining the microenvironment within the brain. This proposal seeks to test select aspects of this general hypothesis through the functional characterization of a novel and large set of gp120 clones derived from primary HIV-1 strains isolated from brain specimens from a cohort of AIDS patients with dementia. These studies will first define the receptor properties of these envelopes in order to delineate their routes of cellular infection. We will then evaluate the properties of these envelopes with regard to selective tropism in brain cell cultures, including a previously- described, three-dimensional heteroaggregate brain culture system. Finally, we will determine the pathobiologic consequences of infection in these cultures, including direct and indirect pathways of toxicity. Collectively these integrated studies will provide essential new information about the role of envelope glycoproteins in directing HIV variants toward specific brain cell types, and the ensuing pathologic events that may contribute to impairment in neurologic function.